a) Field of the Invention
The present invention relates to a method for stereoscopically displaying a topographic map for a navigation system, the navigation system, and a recording medium for recording stereoscopically topographic display program of performing a stereoscopic display processing of the topographic map which is capable of being mounted in the navigation system.
b) Description of the Related Art
In a case where a perspective projection representation for a stereoscopically topographic map is performed in a navigation system using a computer, the computer reads a three-dimensional road map data distributed by a predetermined density in a grid (lattice) form from a three-dimensional road map information recording unit constituted by a CD-ROM, PD (Phase optical Disc), MO disc, DVD, or another large-capacity type recording medium performs a perspective projection transformation with quadrilaterals having vertices of altitude data on four mutually adjoining points of (terrain) locations as three-dimensional graphics to derive data on graphical drawing data in a form of the perspective projection representation (from a single point, viz., a viewpoint and, hereinafter, also called a bird's eye view) on an image screen of a display unit.
However, since, in the case of the bird's eye view representation, a first region to be displayed which is near to the viewpoint is relatively expanded and a second region to be displayed which is remote from the viewpoint is relatively reduced in scale, a quantity of calculations to perform the perspective projection transformation becomes tremendous in addition to a large number of points of locations, especially, at a position remote from the viewpoint when the computer reads the three-dimensional data at constant intervals of the grids not dependently upon a distance from the viewpoint and performs the perspective projection transformation therefor.
A Japanese Patent Application First Publication No. Heisei 5-101163 published on Apr. 23, 1993 exemplifies a previously proposed method for performing a three-dimensional graphic display.
In the above-identified Japanese Patent Application First Publication, each interval between the grids is varied according to its distance from the viewpoint so that the intervals of the grids at the second region are elongated (becomes coarse).
Therefore, the number of points of terrain locations to be displayed within the bird's eye view representation are reduced so that the quantity of calculations are reduced.
Consequently, a speed of displaying the bird's eye view representation on the image screen of the display unit can be quickened.